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ILCD Handbook: Framework and requirements for LCIA models and indicators First edition 1. VOCs or CO react with hydroxyl radical OH• in the troposphere and form peroxy radicals, ROO•. 2. The peroxy radicals oxidize NO to NO2. 3. NO2 is split by sunlight with formation of NO and release of oxygen atoms. 4. Oxygen atoms react with molecular oxygen, O2, to form ozone. Both VOCs and nitrogen oxides are therefore needed for the photochemical ozone formation and should be covered by the characterisation models. The heterogeneous spatial distribution of VOC and NOx sources across Europe, and the hundreds of chemical species involved, makes the photochemical formation of ozone on a regional scale highly non-linear and dynamic. It is influenced by meteorological conditions and interaction between the different VOCs – both from anthropogenic and natural sources, such as forests. The complexity and the number of individual substances for which characterisation factors must be calculated leads to a need for simplification which is obtained in two different ways in the available characterisation models. 1. The non-linear and dynamic behaviour is ignored in a model which represents one or more typical situations in terms of meteorology, atmospheric chemistry and concomitant emissions of other air pollutants. 2. The variation between individual VOCs is (largely) ignored and only a few substance-specific characterisation factors are calculated. The first approach is adopted in the models based on the POCP (Photochemical Ozone Creation Potential) or MIR (Maximum Incremental Reactivity) concept. Here individual characterisation factors are provided for many different VOCs. The second approach is adopted in regionally differentiated models which attempt to capture the non-linear nature of the ozone formation with its spatially and temporally determined differences. Due to the complexity of the underlying chemical reaction schemes and the number of different substances which contribute to photochemical ozone formation, a trade off exists between the degree of detail which can be applied in the fate modelling (including the support of spatially explicit modelling) and the degree of detail applied in the distinction of differences in substance characteristics for the individual VOCs. The variation in photochemical ozone formation between substances is rather modest, except for halogenated hydrocarbons, CH4 and CO, which all have relatively low ozone formation potentials. This is revealed by the POCP or MIR values applied for substance differentiation in several methods. The variation caused by spatial differentiation in the modelling of fate and exposure within Europe is considerably higher (Hauschild et al. 2006). Various studies including those by Andersson-Skjöld in the 1990s seemed to point at a weakness in the calculations of Photochemical Ozone Creating Potential (POCPs) performed in the 1990s using highly detailed chemical mechanisms. The POCPs were generally obtained using very simplified Lagrangian transport models, using linear trajectories, and the results were thus strongly linked to the chemical regimes that the air parcels were passing in the performed scenario calculations. Although the study of Derwent et al. (1998) was performed using a highly detailed chemical mechanism, these new studies indicated that very different results might have been obtained for a different air parcel. It is thus considered preferable to simplify the model on the substance side rather than on the modelling of the dynamic and non-linear nature of the impact pathway. To ensure consistency with several other impact categories, the ideal midpoint indicator would be the time- and area-integrated concentration increase for ozone in the troposphere. This midpoint would cover impacts later in the environmental mechanism on the areas of protection (AoP) ‗Human Health‘ and ‗Natural Environment‘ (vegetation). 4 Requirements for specific impact categories 52
ILCD Handbook: Framework and requirements for LCIA models and indicators First edition 4.6.1.1 Environmental Mechanism (cause-effect chain) Figure 4-8 shows the cause-effect chain for photochemical ozone formation from airborne emissions of volatile organic compounds, carbon monoxide or nitrogen oxides with the most important pathways highlighted (bold arrows). Figure 4-8 Flow diagram for photochemical ozone formation 4.6.2 Criteria for Evaluation of this impact category Next to the general criteria described in Chapter 2, the main criteria ‗Environmental relevance‘ and ‗Scientific robustness‘ have been specified by the following sub criteria: Atmospheric fate and transport is considered. For damages on vegetation, a fate sensitivity factor discriminating between sensitive and insensitive areas is included. For damages on Human Health, a fate sensitivity factor discriminating between sensitive and insensitive areas is included. Magnitude of exceedance for exposure above critical level is considered. Covers both VOCs and inorganic pollutants. Distinction of individual VOCs. 4 Requirements for specific impact categories 53
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